Onion seed viability during storage and drying declines with genotype specificity. Changes in antioxidant enzymes at different viability levels were investigated in seeds of the three onion cultivars 'Akgun-12', 'Valencia' and 'TEG-502'. For this purpose, 95, 80, 60, 40 and 20% viability levels were obtained trough the use of controlled deterioration treatments in seeds of each onion cultivar. Subsequently, catalase (CAT), peroxidase (POX) and superoxide dismutase (SOD) activity measurements were conducted at these viability levels. POX activity was below the detection limit in aged onion seeds. The activities of CAT and SOD decreased due to seed ageing in each cultivar. The decreases observed in CAT and SOD activities were more evident when seed viability decreased below a 60% level. Moreover, a high level of correlation was found between the loss of seed viability and the decreases that occured in CAT and SOD activities, in the seeds,